Deep Ocean Floor Can Focus Tsunami Waves

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As the waves of a tsunami approach a coastline, the topography of
the seafloor near the coast plays a major role in determining how
large those waves become and what places get hit harder than
others.

For example, when the waves of the massive tsunami generated by
last year's magnitude 9.0 Japan earthquake crossed the Pacific
Ocean and reached the U.S. West Coast, they
hit Crescent City, Calif., particularly hard because of two
features of the seafloor off the coast: a piece of the ocean
floor raised by tectonic activity that runs directly toward the
city and the position and shape of the city's harbor.

Scientists had suspected that the same phenomenon might also take
place in the deep ocean, where underwater mountains, called
seamounts, chasms and even islands could deflect tsunami waves in
some places and amplify them in others.

But measurements taken by satellites passing over the waves of
last year's tsunami have confirmed that this happens, even at
large distances from a quake's epicenter.

Researchers from NASA's Jet Propulsion Laboratory and Ohio State
University used satellite altimeters, which can measure sea level
changes in very fine detail, to observe " merging
tsunamis " — a phenomenon where smaller waves merge to form
one bigger wave. These waves can travel hundreds to thousands of
miles without losing power.

The measurements showed that the
March 2011 tsunami doubled in intensity when passing over
rugged ocean ridges and around islands in the middle of the
Pacific.

The team used a computer-based model to translate the
measurements into images and animations, which shows how the
waves can refract, bend and merge as they propagate. The peaks of
waves are colored red-brown, while depressions in sea surface
appear blue-green. Grayscale outlines show the location of
mid-ocean ridges, peaks and islands.

The measurements came from the Jason-1, Jason-2, and Envisat
satellites, which each flew over the tsunami at a different
location.

"It was a one in 10 million chance that we were able to observe
this double wave with satellites," said Tony Song, principal
investigator of the study and a scientist at JPL.

"Researchers have suspected for decades that such ‘merging
tsunamis’ might have been responsible for the 1960 Chilean
tsunami that killed about 200 people in Japan and Hawaii, but
nobody had definitively observed a merging tsunami until now. It
was like looking for a ghost," Song said. "Jason happened to be
in the right place at the right time to capture the double wave."